3. CONTENTS. . .
Porosity and its evaluation tests
True Density
Bulk Density
Numerical problems and solutions
Bulkiness
Flow properties of powders
Factors affecting on flow properties
4. Porosity:
The porosity or voids is defined as
the ratio of the void volume to bulk volume.
𝜀 =(VB −𝑉P )/ Vb
=1- Vp / Vb
Void volume = Vb – Vp
Porosity is expressed in percent, 𝜀 × 100
Vb=Bulk volume
Vp=True volume
5. Problems:
I. A sample of calcium oxide powder with a true density of
3.203 and weighing 131.3 gm was found to have a bulk
volume of 82cm3 when placed in a 100 ml graduated cylinder
.calculate the porosity.
The volume of the particle is (131.3g /3.203gcm-3 ) =41cm3
𝜀 =(vb- vp)/ vb
=(82-41)/ 82
=0.5× 100=50%
6. Densities of Powders:
It is defined as the mass per unit
volume.
Types of density:
There are three types of densities based
on the determination method :
True Density.
Bulk Density.
Granule Density.
7. True Density (𝝆):
Defined as the volume of the pores
and voids within the sample.
Bulk Density (𝝆b)
Defined as the volume of all of the
pores within the sample.
Granule Density(𝝆g)
Defined as the ratio of the mas of the
granular powder and granule volume.
8. Bulkiness:
The reciprocal of bulk density
is known as the bulkiness or
specific bulk volume.
• It increases with a decreases in
particle size.
• It may get reduced since the
smaller particles may shift
between the larger ones.
9. Flow properties:
Flow properties of powder is in an
important parameter in the production of
pharmaceutical dosage forms. It may be free
flowing or may have poor flow.
Reasons of poor flow:
• Cohesiveness between particles due to
van der waals, electrostatic forces.
• Adhesion between the particles and the
container wall.
• Friction between particles.
10. Test to evaluate the flow ability
of a powder
Carrs Compressibility Index:
A volume of powder is filled into a graduated glass
cylinder and repeatedly tapped a known duration.The volume of
powder after tapping is measured.
Carrs Index (%) = Tapped density- Bulk density
Bulk density =Weight/bulk volume
Tapped density=Weight/ true volume
11. Relationship between powder flowability and %
compressibility:
• Flow Description Compressibility
Excellent flow 5-15
Good 16-18
Fair 19-21
Poor 22-35
Very poor 36-40
Extremely poor > 40
12. Hausner Ratio:
H.ratio =Tapped density / Bulk density
• It is related to inter particle friction.
• Values less than 1.25 indicates good flow ( coarse
spheres) and greater than 1. 5 indicates poor flow .(
flakes)
• Between 1.25 and 1.5 added glidant normally improves
flow.
• > 1.5 added glidant does not improve flow.
13. Factors affecting the flow properties of powders
• Particle size distribution: ↑ the average particle size improves the flow
properties due to reduction in the cohesive forces.
• Particle shape and texture: spherical particles have better flow ability
as compared to irregular particles.
• Surface forces: Reduction of electrostatic charges can improve powder
flow ability .Drying of particles will reduce the cohesiveness and ↑
𝑓𝑙𝑜𝑤.
• Addition of fines: presence of small portion of fines in a powder may
improve the flow properties.
• Flow activators:↑
𝑡ℎ𝑒 𝑓𝑙𝑜𝑤𝑎𝑏𝑖𝑙𝑖𝑡𝑦 𝑏𝑦 𝑟𝑒𝑑𝑢𝑐𝑖𝑛𝑔 𝑎𝑑ℎ𝑒𝑠𝑖𝑜𝑛 𝑎𝑛𝑑 𝑐𝑜ℎ𝑒𝑠𝑖𝑜𝑛.
14. REFFERENCE…
Micromeritics and powder Rheology ;Physical
Pharmacy; S.P Agarwal, Rajesh Khanna; page no-
38,39,40,.41,42,43,44,45;CBS publishers; Second
edition.
